Exact current measurements are of central importance in, electrical engineering. In this regard, e.g., current measurements in control loops generally directly determine a quality of a respective overall system. A so-called Rogowski-Steinhaus-Chattock coil can be used for measuring electrical AC current flowing m a measurement object, said coil allowing measurements with very high bandwidths.
A Rogowski-Steinhaus-Chattock current sensor consists of at least one Rogowski-Steinhaus-Chattock coil constructed from a toroidal conductor loop embodied in a circular fashion, in order to arrange the Rogowski-Steinhaus-Chattock coil perpendicularly around a conductor in a line system, the circle can have an opening. The return line of the toroidal conductor loop ending at the opening is usually led through the interior of the Rogowski-Steinhaus-Chattock coil. Rogowski-Steinhaus-Chattock coils constitute a simple and reliable method of ascertaining a current flow in an electrical line system. The current flow to be measured causes a magnetic field which surrounds the conductor and which induces a voltage in a Rogowski-Steinhaus-Chattock coil situated around the conductor. The current in the conductor can then be calculated from said voltage. Rogowski-Steinhaus-Chattock current sensors are used in a wide frequency range of AC currents. In this case, measurement disturbances become more and more significant toward higher frequencies.
As mentioned above, a Rogowski-Steinhaus-Chattock coil consists of a measuring line which is wound to form a toroid and which in general is wound as an outgoing conductor in the form of a helix spiral and after a turning point is led as a return conductor within the toroid back to a beginning of the toroid. The Rogowski-Steinhaus-Chattock coil (see A. P. Chattock (1887), “On a magnetic potentiometer”, Proceedings of the Physical Society of London on 23-26, which is incorporated by reference herein; W. Rogowski and W. Steinhaus (1912), “Die Messung der magnetischen Spannung; Messung des Linienintegrals der magnetischen Feldstärke” [“Measuring the magnetic potential difference: measuring the line integral of the magnetic field strength”, Archiv für Elektrotechnik, 1(4): 141-150), which is incorporated by reference herein, uses a toroidal coil to be placed around a measurement current or around a conductor carrying the measurement current. In a departure therefrom, the coil need not necessarily enclose the conductor (see S. Hain, M. Bakran (2014), “Highly dynamic current measurements with inductive current sensors—a numerical recipe”, PCIM Europe, 1617-1624), which is incorporated by reference herein. In order to measure, by means of a Rogowski-Steinhaus-Chattock coil, a voltage signal that is proportional to the derivative of an electric current to be measured, the Rogowski-Steinhaus-Chattock coil should be wound very uniformly.
The Rogowski-Steinhaus-Chattock coil allows a measurement of AC currents with very high bandwidths from the millihertz range to the megahertz range. Using conventional Rogowski-Steinhaus-Chattock coils for current measurement, however, in the case of an AC current having a frequency of a few megahertz, limitations occur on account of physical properties of the Rogowski-Steinhaus-Chattock coil. It has been found that particular electrical capacitances occurring within a respective Rogowski-Steinhaus-Chattock coil and between the Rogowski-Steinhaus-Chattock coil and a measurement object or ground influence a measurement accuracy of a current measured by means of the Rogowski-Steinhaus-Chattock coil. In particular, four types of electrical capacitances which affect the measurement accuracy of a Rogowski-Steinhaus-Chattock coil can be identified here.
In this regard, an electrical capacitance between two arbitrary, in particular adjacent, turns of a Rogowski-Steinhaus-Chattock coil occurs in particular at a high frequency of an AC current passing through the Rogowski-Steinhaus-Chattock coil. This electrical capacitance has a greater effect, the higher a voltage difference between respective turns becomes, since a coupling between the capacitance and a voltage difference between the respective turns takes effect. Such a voltage difference increases with a frequency of a respective AC current passing through the coil as soon as, on account of short wavelength of the AC current, voltage differences already form between adjacent turns of the Rogowski-Steinhaus-Chattock coil. Furthermore, a voltage difference between respective turns of a Rogowski-Steinhaus-Chattock coil increases with a reduction of a spatial distance between respective turns. In order to increase a sensitivity of a respective Rogowski-Steinhaus-Chattock coil, however, the distance between respective turns of the Rogowski-Steinhaus-Chattock coil is often reduced or kept as small as possible and a number of turns is increased, or chosen to be as high as possible, such that an occurrence of capacitances between the turns in promoted in accordance with the explanations given above.
Analogously to the electrical capacitance between a respective turn of a Rogowski-Steinhaus-Chattock coil, an electrical capacitance between the respective turn and a return conductor led in the interior of a Rogowski-Steinhaus-Chattock coil can occur and be considered equivalently to the electrical capacitance between two arbitrary turns.
Furthermore, an electrical capacitance arises between each individual conductor part of a Rogowski-Steinhaus-Chattock coil and a respective measurement object or ground. In this case, an influence of a coupling between a respective conductor part of the Rogowski-Steinhaus-Chattock coil and the measurement object increases with a voltage and a frequency of an electric current flowing in the measurement object.
If an electrostatic shielding s present (see e.g. C. Hewson, W. F. Ray (2004), “The effect of electrostatic screening of Rogowski coils designed for wide-bandwidth current measurement in power electronic applications”, Annual IEEE Power Electronics Specialists Conference, 35:1143-1148), which is incorporated by reference herein, a coupling between each individual conductor part of a Rogowski-Steinhaus-Chattock coil and a shielding potential is furthermore present.
In addition, all capacitances of a respective Rogowski-Steinhaus-Chattock coil together with a respective inductance form filters or resonances that can reduce a measurement bandwidth by orders of magnitude. Generally, an impedance across a capacitance decreases with increasing frequency of an AC current to be measured, for which reason signals and interference can undergo greater crosstalk in particular at high frequencies across capacitances.
What is important for the understanding and the function is to clearly differentiate between the voltage in a conductor, for example between the ends of the Rogowski-Steinhaus-Chattock coil which supply the measurement signal, and a reference potential of the conductor relative to ground. In this case, the voltage is defined merely as a difference and forms in particular as a result of the induction by the AC current to be measured. In this case, the voltage between two points of a conductor corresponds by definition to the difference between the potentials of the two points. However, the voltage, including the voltage induced by the current to be measured, does not define the potential of the corresponding conductor. However, this potential is of central importance for minimizing the capacitive coupling.
The document WO 2015 104 189 A1, which is incorporated by reference herein, describes a current measuring device that provides a Rogowski coil having two shields. In this case, the Rogowski coil is wound around a first shield, with a second shield surrounding the Rogowski coil. As a result of the shields, the measurement inaccuracies of the Rogowski coil are reduced and the bandwidth is increased.
The document WO 2010 041 139 A1, which is incorporated by reference herein, describes a coil for current measurement in electrical conductors, wherein the coil comprises at least one geometrically fully closed loop, that is to say is not embodied in a spiral fashion, in contrast to routine practice in the case of Rogowski coils. Adjacent loops here are electrically connected to one another.
Document EP 2 084 721 B1, which is incorporated by reference herein, describes a Rogowski coil arrangement comprising two Rogowski coils surrounded respectively by a magnetic shield, wherein a relay subtracts the voltage signals of the two coils from one another in order to minimize electrical disturbances.
Document DE 35 44 508 A1, which is incorporated by reference herein, describes a combined transducer for simultaneously measuring current and voltage on a conductor, wherein a Rogowski coil surrounded by a metallic, nonmagnetic shield is provided. In this case a capacitor is formed by the shield and the conductor to be measured, said capacitor being used for measuring the voltage in the conductor.
The document U.S. Pat. No. 7,545,138 B2, which is incorporated by reference herein, discloses a current measuring device in which a coil conductor is wound once completely around a coil core and is then wound completely in the opposite direction. In addition, the measuring current is surrounded by a Faraday shield.